In the current state of computer technology, image reconstruction has a wide spread use. This is particularly true for medical-diagnostic imaging, for example by computer tomography (CT). Computer tomography is also utilized for the non-destructive testing of materials, for example in the automotive industry.
In a computer tomography system, an X-ray source is collimated to form a fan beam with a defined fan beam angle and fan beam width. The fan beam is oriented to lie within the X-Y plane of a Cartesian coordinate system, termed the “imaging plane”, and to be transmitted through an imaged object to an X-ray detector array oriented within the imaging plane. The detector array is comprised of detector elements, which each measure the intensity of transmitted radiation along a ray projected from the X-ray source to that particular detector element. The intensity of transmitted radiation received by each detector element in the detector array is dependent on the attenuation of the X-ray beam along a ray by the imaged object. Each detector element produces an intensity signal dependent on the intensity of transmitted radiation stirring the detector element. The X-ray source and detector array may be rotated on a gantry within the imaging plane so that the fan beam intercepts the imaged object at different angles. At each angle, a projection is acquired, comprised of the intensity signals of each of the detector elements. The projection at each of these different angles together form a tomographic projection set. Normally a projection set will be taken over 360° of gantry rotation. An attempt to reconstruct an image with less than a complete projection set will normally lead to image artifacts caused by the missing data and image blurring.
In many practical applications a free access to an image object by an X-ray acquisition system in order to acquire a complete projection set is not possible due to a limited time for data acquisition or the limited spatial accessibility of the image object.
One example is verification imaging in radiation therapy. The main object of radiotherapy is to deliver the prescribed dosage of radiation to a tumor in a patient while minimizing the damage to surrounding, healthy tissue. Since very high energy radiation is normally used to destroy tumors in radiotherapy, the high energy is also destructive to the normal tissue surrounding the tumor. Therefore, it is essential that the delivery of radiation be limited precisely to the prescribed target volume (i.e. the tumor plus adequate margins). The main sources of the problem result from the fact that there is a natural motion of organs inside the body, which can range from approximately a millimetre in the case of the brain inside the skull to several centimetres for the organs in the trunk. Another factor relates to changes which occur in the tumor over time as a result of successful treatment. As the tumor shrinks in volume, normal tissue which had been displaced returns to its original position within the treatment volume. Furthermore, patient movement can be another reason for positioning errors during irradiation. To accurately verify tumor positioning, one method taught in the prior art is to utilize a low dose, low energy X-ray source in conjunction with the therapy beam source.
U.S. Pat. No. 5,233,990 A relates to a method for producing corrected verification images of anatomical portions of a patient being treated with conventional radiation therapy equipment consisting of a gantry having a high-energy radiation source capable of emitting a high-energy radiation beam along a high energy beam axis, said gantry being disposed for irradiation of a stationary patient positioned on a gurney at a fixed distance from the high-energy radiation source. The method comprises the steps of
providing a low-energy radiation source mounted on a side of the gantry opposite to the high-energy radiation source, said low-energy radiation source being capable of emitting a low-energy radiation beam along a low-energy beam axis coaxial with the axis of the beam from the high-energy radiation source;
providing a radiation detector mounted between the high-energy radiation source and the patient's gurney, said detector being positioned so that it can be exposed to both the high-energy beam and the low-energy beam;
irradiating the patient with said coaxial low-energy radiation source during treatment to form verification images on said radiation detector and
comparing the verification images with the diagnostic images during treatment to ensure that the anatomical portions of the patient being irradiated by high-energy radiation treatment correspond to the anatomical portions delineated on the diagnostic images.
WO 03/076016 A1 is directed to a device for performing and verifying therapeutic radiation. An X-ray is arranged across from a target volume of the beam source for a high-energy beam in such a way that the beams run in essentially opposite directions. The invention also relates to a computer program and a control method for operating said device. The inventive device makes it possible to exactly verify areas that are subjected to different levels of radiation, the entire anatomy of the target volume and the surroundings thereof in addition to the contour of the therapy beam. The X-ray detects the anatomy and position of the patient within the range of the target volume before the high-energy beam is applied and then detects the shape of the applied high-energy beam and areas that are subjected to different levels of radiation as well as at least one partial segment of the target volume during the emission breaks of the high-energy beam. The detected data is used for correcting the treatment plan.
The prior art systems for radiation therapy contain a limited accessible space for the placement of the verification imaging equipment. Furthermore, there is little time during the emission breaks of the therapy beam to collect data for the verification imaging. Therefore no complete data sets can be acquired. With an incomplete data set, an image reconstruction with reasonable quality is not possible using conventional image reconstruction techniques.